The present invention relates to treads for tires intended to be fitted on the front axle of transport vehicles and more particularly heavy vehicles, for example, long haul trucks for highway use.
Tires for heavy vehicles typically comprise a carcass reinforcement comprising a plurality of reinforcements arranged radially, the carcass reinforcement itself being surmounted by a crown reinforcement extending in the circumferential direction. This crown reinforcement is composed of at least two plies superposed on one another, each ply being formed of a rubber mix reinforced by a plurality of cables or cords of low extensibility, preferably made of steel, arranged parallel to each other in one and the same ply and in a direction forming an angle with the tire circumferential direction at most equal to 40°, the cables from one ply to the other being crossed between each other. This crown reinforcement may further include, on one hand, two half-plies formed of cables of low extensibility forming with the circumferential direction an angle of between 45° and 80°, and on the other hand, at least one ply formed of so-called “elastic” cables placed radially to the outside of the crown plies, the reinforcement cables of which form an angle of less than 40°.
The crown reinforcement is furthermore surmounted on its radially outer surface by a tread formed with at least one rubber mix, the radially outer surface of which forms the running surface intended to come into contact with the roadway during travel of said tire.
In order to obtain satisfactory grip performance during travel on a roadway covered with water, the tread is provided on its outer surface, in the case of tires intended for fitting on the front steering axle of heavy vehicles, with a plurality of grooves of generally circumferential orientation. These grooves form a tread pattern comprising a plurality of ribs, each rib having a contact face radially to the outside and lateral walls which may be substantially perpendicular to the contact face of the rib or alternatively which may form a taper angle other than zero with this face. To improve the grip still further, it is known to provide some of the ribs with a plurality of grooves and/or incisions of transverse orientation. These transverse cutouts and the longitudinal grooves define a plurality of rubber pattern elements having during travel a leading edge or ridge, that is to say an edge which comes into contact with the roadway before the rest of the contact surface of said element, and a trailing edge. The appearance of what is called “irregular” wear, as it develops on the contact surface of these elements in the vicinity of the trailing edge during travel of the tire provided with this tread was noted.
In order to improve the resistance to this irregular wear of this tread by reducing the tendency to irregular wear on the surfaces located close to the trailing edges, the Applicant proposed a new tread pattern described in European Patent EP 0384182 (U.S. Pat. No. 5,896,905).
According to this document, there is proposed a tread pattern comprising circumferential grooves so as to provide the tread with at least five ribs, in which at least the ribs defined by two circumferential grooves being provided with incisions of a width of less than 3 mm, which are transverse and substantially parallel to each other, these incisions being inclined by an angle of between 5° and 25° relative to a direction perpendicular to the running surface of the tread such that the resulting force exerted, during travel in the zone of contact with the ground, by the ground on the tread tends to straighten the incisions towards a zero inclination relative to said perpendicular direction.
Although this arrangement makes it possible effectively to combat this type of irregular wear, it will be noted that the overall average wear of the edge ribs is substantially increased relative to the average overall wear of the intermediate ribs. This difference in wear is, after a certain mileage traveled, the cause of the appearance of what is called “rail” irregular wear on the longitudinal edges of the edge ribs; this wear is linked to the development of braking stresses exerted by the roadway on these edge ribs resulting from the differences in lengths traveled by the edge ribs and the intermediate ribs.
Simplified, the wear becomes more regular but the wear life of the tread is reduced compared with that of a tire having a tread which is not provided with inclined incisions. “Wear life” is understood to mean the possible period of use before complete wear of a tread of a tire during travel, resulting either in the changing of the tire or in the renewal of the tread by recapping.
There is therefore a need for a tread pattern for tires which are intended to be fitted on the front axle of heavy vehicles which has no (or only little) irregular wear and substantially the same rate of wear for all the ribs, while maintaining a low average rate of overall wear, these performances imparting to the tire an improved life compared with the tire of the European application previously mentioned.